cpu: Fix the usage of const DynInstPtr

Summary: Usage of const DynInstPtr& when possible and introduction of
move operators to RefCountingPtr.

In many places, scoped references to dynamic instructions do a copy of
the DynInstPtr when a reference would do. This is detrimental to
performance. On top of that, in case there is a need for reference
tracking for debugging, the redundant copies make the process much more
painful than it already is.

Also, from the theoretical point of view, a function/method that
defines a convenience name to access an instruction should not be
considered an owner of the data, i.e., doing a copy and not a reference
is not justified.

On a related topic, C++11 introduces move semantics, and those are
useful when, for example, there is a class modelling a HW structure that
contains a list, and has a getHeadOfList function, to prevent doing a
copy to an internal variable -> update pointer, remove from the list ->
update pointer, return value making a copy to the assined variable ->
update pointer, destroy the returned value -> update pointer.

Change-Id: I3bb46c20ef23b6873b469fd22befb251ac44d2f6
Signed-off-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/13105
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

1 files changed, 7 insertions, 9 deletions

diff --git a/src/cpu/o3/commit_impl.hh b/src/cpu/o3/commit_impl.hh
index d32493cbc..8fd142c08 100644
--- a/src/cpu/o3/commit_impl.hh
+++ b/src/cpu/o3/commit_impl.hh
@@ -636,7 +636,7 @@ DefaultCommit<Impl>::squashFromSquashAfter(ThreadID tid)
 
 template <class Impl>
 void
-DefaultCommit<Impl>::squashAfter(ThreadID tid, DynInstPtr &head_inst)
+DefaultCommit<Impl>::squashAfter(ThreadID tid, const DynInstPtr &head_inst)
 {
     DPRINTF(Commit, "Executing squash after for [tid:%i] inst [sn:%lli]\n",
             tid, head_inst->seqNum);
@@ -696,14 +696,14 @@ DefaultCommit<Impl>::tick()
             // will be active.
             _nextStatus = Active;
 
-            DynInstPtr inst = rob->readHeadInst(tid);
+            const DynInstPtr &inst M5_VAR_USED = rob->readHeadInst(tid);
 
             DPRINTF(Commit,"[tid:%i]: Instruction [sn:%lli] PC %s is head of"
                     " ROB and ready to commit\n",
                     tid, inst->seqNum, inst->pcState());
 
         } else if (!rob->isEmpty(tid)) {
-            DynInstPtr inst = rob->readHeadInst(tid);
+            const DynInstPtr &inst = rob->readHeadInst(tid);
 
             ppCommitStall->notify(inst);
 
@@ -1136,7 +1136,7 @@ DefaultCommit<Impl>::commitInsts()
 
 template <class Impl>
 bool
-DefaultCommit<Impl>::commitHead(DynInstPtr &head_inst, unsigned inst_num)
+DefaultCommit<Impl>::commitHead(const DynInstPtr &head_inst, unsigned inst_num)
 {
     assert(head_inst);
 
@@ -1317,9 +1317,7 @@ DefaultCommit<Impl>::getInsts()
     int insts_to_process = std::min((int)renameWidth, fromRename->size);
 
     for (int inst_num = 0; inst_num < insts_to_process; ++inst_num) {
-        DynInstPtr inst;
-
-        inst = fromRename->insts[inst_num];
+        const DynInstPtr &inst = fromRename->insts[inst_num];
         ThreadID tid = inst->threadNumber;
 
         if (!inst->isSquashed() &&
@@ -1366,7 +1364,7 @@ DefaultCommit<Impl>::markCompletedInsts()
 
 template <class Impl>
 void
-DefaultCommit<Impl>::updateComInstStats(DynInstPtr &inst)
+DefaultCommit<Impl>::updateComInstStats(const DynInstPtr &inst)
 {
     ThreadID tid = inst->threadNumber;
 
@@ -1507,7 +1505,7 @@ DefaultCommit<Impl>::oldestReady()
 
             if (rob->isHeadReady(tid)) {
 
-                DynInstPtr head_inst = rob->readHeadInst(tid);
+                const DynInstPtr &head_inst = rob->readHeadInst(tid);
 
                 if (first) {
                     oldest = tid;